In the manufacture of disposable absorbent articles such as diapers and feminine hygiene, it is a common manufacturing practice to combine continuously moving webs. These webs are typically represented by plastic films, non-wovens, elastics, etc. supplied to a converting line in their original production form and do not require special positioning before combining with other materials on the converting line. However, the manufacture of disposable absorbent articles may benefit if the above materials are supplied to the converting line as pre-produced materials, i.e., produced off the converting line and carrying various elements of disposable absorbent articles consecutively spaced along the web length at a nominal pitch length. Because the pitch length between the consecutively spaced elements of the pre-produced materials can vary at small but significant variations, commonly understood as random disturbances, in order to ensure consistent positioning of the product elements in every absorbent article, there is a demand for a method to register the consecutively spaced pre-produced elements in relation to desired positions, combining the pre-produced materials into a final article or product.
For example, a pre-produced breathable polymer film made off the converting lines that are particularly useful as backsheet materials for disposable absorbent articles typically have good surface characteristics that make them suitable for the application of multi-colored, high resolution graphics, but such films tend to be mechanically unstable with a particular tendency toward thermal shrinkage in the machine direction, commonly understood as a steady state disturbance. Such instabilities contribute to the difficulty in correctly positioning the pre-produced objects (e.g. graphics) on the polymer web in relation with a desired, target position on a disposal absorbent article. Various methods and apparatus have previously been used for combining components having random disturbances. For example, in conventional “discrete phasing” operations, the product is built around the pre-produced object, by adjusting the timing position of the machinery in response to a correction signal. However, it cannot be used in operations where in webs having steady state disturbance described above. See, e.g., International Publication No. WO 96/29966, and U.S. Pat. No. 5,659,538.
In “non-continuous web placement” operations, the pre-produced web is discretized, or cut, into segments. Each segment is then placed on the target web or product in the desired position. This does not, however, ensure the centering of the pre-produced object on the target web; furthermore, it limits the overall product design. The length of the discretized segments must be less than the length of the product, which may allow leakage in the areas where the product is longer than the discretized segment, for example, in the waist region of a diaper.
Other control systems and apparatus for registration have been described; see International Publications WO 97/24094 and WO 97/24283. However, none of the existing systems or apparatus for registration addresses a system in which inherent significant material instabilities exist, nor do they address a system in which the pitch length of the pre-produced object is non-uniform, particularly if the pre-produced pitch length is longer than the pitch length of the product under production. Similarly, a control system such as that disclosed in U.S. Pat. No. 5,766,389 requires a fixed target pitch. In practice, the target pitch can be subject to the same instabilities as that of the pre-producing webs.
Based on the foregoing, there is a need for a system for registering pre-produced webs having variable pitch lengths, at least one of which has consecutively spaced pre-produced objects. Furthermore, there is a need for a registration control system that can account for the material specific instabilities that inherently exist in such webs and that present small but significant variations resulting there from, which make it difficult to work (e.g., combine) such webs. There is also a need for a system in which registration of incoming webs having either longer or shorter pitch lengths than the pitch length of the product under production is possible. In addition, there is a need for a system in which automatic phase control is needed for the combination of the random pitch disturbance and steady state disturbance which result in a mean shift in the pre-produced pitch around a target pitch length which make it difficult to work such webs. None of the existing systems provides all of the advantages and benefits of the present invention.